The hypothalamus-pituitary-gonad axis: Tales of mice and men

Comparative Transcriptomics Identify Key Hypothalamic Circular RNAs that Participate in Sheep (Ovis aries) Reproduction

Simple Summary

The hypothalamus plays crucial roles in sheep reproduction. However, the expression profiles of sheep hypothalamic circular RNA (circRNA), which has been proved to exert important functions in many physiological processes, remain largely unknown. In this study, we used RNA sequencing to explore the expression of circRNAs in the hypothalamus of sheep with the FecB ++ genotype. The results suggested that several key hypothalamic circRNAs may participate in sheep reproduction by influencing gonadotropin-releasing hormone (GnRH) activities or affecting key gene expression indirectly or directly. This study provides a further reference for understanding the differences of sheep fecundity.

Abstract

Circular RNA (circRNA), as an emerging class of noncoding RNA, has been found to play key roles in many biological processes. However, its expression profile in the hypothalamus, a powerful organ initiating the reproductive process, has not yet been explored. Therefore, we used RNA sequencing to explore the expression of circRNAs in the hypothalamus of sheep with the FecB ++ genotype. We totally identified 41,863 circRNAs from sheep hypothalamus, in which 333 (162 were upregulated, while 171 were downregulated) were differentially expressed in polytocous sheep in the follicular phase versus monotocous sheep in the follicular phase (PF vs. MF), moreover, 340 circRNAs (163 were upregulated, while 177 were downregulated) were differentially expressed in polytocous sheep in the luteal phase versus monotocous sheep in the luteal sheep (PL vs. ML). We also identified several key circRNAs including oar_circ_0018794, oar_circ_0008291, oar_circ_0015119, oar_circ_0012801, oar_circ_0010234, and oar_circ_0013788 through functional enrichment analysis and oar_circ_0012110 through a competing endogenous RNA network, most of which may participate in reproduction by influencing gonadotropin-releasing hormone (GnRH) activities or affecting key gene expression, indirectly or directly. Our study explored the overall expression profile of circRNAs in sheep hypothalamus, which potentially provides an alternative insight into the mechanism of sheep prolificacy without the effects of FecB mutation.

Identification and functional characterization of microRNAs in rat Leydig cells during development from the progenitor to the adult stage

The aim of the present study was to identify microRNAs (miRNAs) that regulate the proliferation and differentiation of Leydig cells (LCs) of rat. Three small RNA libraries derived from progenitor LCs (PLCs), immature LCs (ILCs) and adult LCs (ALCs) were analyzed by microarrays. In total, 68 differentially expressed miRNAs (DEMs) were identified. Based on the trend of DEM expression from PLCs to ALCs, primary LCs were transfected with miRNA mimics or inhibitors. Five miRNAs (miR-30a-5p, miR-3585-5p, miR-212-3p, miR-369-5p and miR-434-3p) promoted PLC proliferation, and 3 miRNAs (miR-17-5p, miR-532-3p and miR-329-3p) activated caspase-3, which triggered LC apoptosis. For steroidogenesis, 18 miRNAs could elevate or inhibit androsterone release at the PLC stage. Eleven and 9 miRNAs inhibited the production of 5α-androstane-3α,17β-diol in ILCs and testosterone in ALCs, respectively. miR-17-5p, miR-29a-3p and miR-299a-5p decreased androgen production by LCs at all developmental stages. Furthermore, the miR-299a-5p-mediated decrease in androgen production by the LC lineage was primarily achieved by downregulating the expression of luteinizing hormone/choriogonadotropin receptor (LHCGR) and 3β-hydroxysteroid dehydrogenase 1 (HSD3B1). These findings provide insights into the regulatory roles of miRNAs during the postnatal development of LCs and suggest potential strategies for the treatment of steroid-related disorders.

Leydig cells: formation, function, and regulation

Herein we summarize important discoveries made over many years about Leydig cell function and regulation. Fetal Leydig cells produce the high levels of androgen (testosterone or androstenedione, depending upon the species) required for differentiation of male genitalia and brain masculinization. Androgen production declines with loss of these cells, reaching a nadir at postpartum. Testosterone then gradually increases to high levels with adult Leydig cell development from stem cells. In the adult, luteinizing hormone (LH) binding to Leydig cell LH receptors stimulates cAMP production, increasing the rate of cholesterol translocation into the mitochondria. Cholesterol is metabolized to pregnenolone by the CYP11A1 enzyme at the inner mitochondrial membrane, and pregnenolone to testosterone by mitochondria and smooth endoplasmic reticulum enzymes. Cholesterol translocation to the inner mitochondrial membrane is mediated by a protein complex formed at mitochondrial contact sites that consists of the cholesterol binding translocator protein, voltage dependent anion channel, and other mitochondrial and cytosolic proteins. Steroidogenic acute regulatory protein acts at this complex to enhance cholesterol movement across the membranes and thus increase testosterone formation. The 14-3-3γ and ε adaptor proteins serve as negative regulators of steroidogenesis, controlling the maximal amount of steroid formed. Decline in testosterone production occurs in many aging and young men, resulting in metabolic and quality-of-life changes. Testosterone replacement therapy is widely used to elevate serum testosterone levels in hypogonadal men. With knowledge gained of the mechanisms involved in testosterone formation, it is also conceivable to use pharmacological means to increase serum testosterone by Leydig cell stimulation.

Extragonadal FSHR Expression and Function-Is It Real?

Expression of the follicle-stimulating hormone receptor (FSHR), besides gonadal tissues, has recently been detected in several extragonadal normal and tumorous tissues, including different types of primary and metastatic cancer and tumor vessel endothelial cells (TVEC). The suggested FSH actions in extragonadal tissues include promotion of angiogenesis, myometrial contractility, skeletal integrity, and adipose tissue accumulation. Non-malignant cells within cancer tissue have been shown to be devoid of FSHR expression, which implies a potential role of FSHR as a diagnostic, prognostic, or even a therapeutic tool. There are shared issues between several of the published reports questioning the validity of some of the conclusion. Firstly, protein expression of FSHR was performed solely with immunohistochemistry (IHC) using either an unavailable "in house" FSHR323 monoclonal antibody or poorly validated polyclonal antibodies, usually without additional methodological quality control and confirmations. Secondly, there is discrepancy between the hardly traceable or absent FSHR gene amplification/transcript data and non-reciprocal strong FSHR protein immunoreactivity. Thirdly, the pharmacological high doses of recombinant FSH used in in vitro studies also jeopardizes the physiological or pathophysiological meaning of the findings. We performed in this review a critical analysis of the results presenting extragonadal expression of FSHR and FSH action, and provide a rationale for the validation of the reported results using additional more accurate and sensitive supplemental methods, including in vivo models and proper positive and negative controls.

Fertility and reproductive care in chronic kidney disease

In both women and men, chronic kidney disease (CKD) is associated with decreased fertility. Though a multitude of factors contribute to the reduction in fertility in this population, progressively impaired function of the hypothalamic-pituitary-gonadal axis appears to play a key role in the pathophysiology. There is limited research on strategies to manage infertility in the CKD population, but intensive hemodialysis, kidney transplantation, medication management and assisted reproductive technologies (ART) have all been proposed. Though fertility and reproductive care are reported as important elements of care by CKD patients themselves, few nephrology clinicians routinely address fertility and reproductive care in clinical interactions. Globally, the average age of parenthood is increasing, with concurrent growth and expansion in the use of ART. Coupled with an increasing prevalence of CKD in women and men of reproductive age, the importance of understanding fertility and reproductive technologies in this population is highlighted. This review endeavors to explore the female and male factors that affect fertility in the CKD population, as well as the evidence supporting strategies for reproductive care.

Modulation of Gonadotropin-Releasing Hormone Neuron Activity and Secretion in Mice by Non-peptide Neurotransmitters, Gasotransmitters, and Gliotransmitters

Gonadotropin-releasing hormone (GnRH) neuron activity and GnRH secretion are essential for fertility in mammals. Here, I review findings from mouse studies on the direct modulation of GnRH neuron activity and GnRH secretion by non-peptide neurotransmitters (GABA, glutamate, dopamine, serotonin, norepinephrine, epinephrine, histamine, ATP, adenosine, and acetylcholine), gasotransmitters (nitric oxide and carbon monoxide), and gliotransmitters (prostaglandin E2 and possibly GABA, glutamate, and ATP). These neurotransmitters, gasotransmitters, and gliotransmitters have been shown to directly modulate activity and/or GnRH secretion in GnRH neurons in vivo or ex vivo (brain slices), from postnatal through adult mice, or in embryonic or immortalized mouse GnRH neurons. However, except for GABA, nitric oxide, and prostaglandin E2, which appear to be essential for normal GnRH neuron activity, GnRH secretion, and fertility in males and/or females, the biological significance of their direct modulation of GnRH neuron activity and/or GnRH secretion in the central regulation of reproduction remains largely unknown and requires further exploration.

Neuropeptide and steroid hormone mediators of neuroendocrine regulation

To maintain the health and well-being of all mammals, numerous aspects of physiology are controlled by neuroendocrine mechanisms. These mechanisms ultimately enable communication between neurones and glands throughout the body and are centrally mediated by neuropeptides and/or steroid hormones. A recent session at the International Workshop in Neuroendocrinology highlighted the essential roles of some of these neuropeptide and steroid hormone mediators in the neuroendocrine regulation of stress-, reproduction- and behaviour-related processes. Accordingly, the present review highlights topics presented in this session, including the role of the neuropeptides corticotrophin-releasing factor and gonadotrophin-releasing hormone in stress and reproductive physiology, respectively. Additionally, it details an important role for gonadal sex steroids in the development of behavioural sex preference.

Diet-Induced Obesity Elicits Macrophage Infiltration and Reduction in Spine Density in the Hypothalami of Male but Not Female Mice

Increasing prevalence in obesity has become a significant public concern. C57BL/6J mice are prone to diet-induced obesity (DIO) when fed high-fat diet (HFD), and develop chronic inflammation and metabolic syndrome, making them a good model to analyze mechanisms whereby obesity elicits pathologies. DIO mice demonstrated profound sex differences in response to HFD with respect to inflammation and hypothalamic function. First, we determined that males are prone to DIO, while females are resistant. Ovariectomized females, on the other hand, are susceptible to DIO, implying protection by ovarian hormones. Males, but not females, exhibit changes in hypothalamic neuropeptide expression. Surprisingly, ovariectomized females remain resistant to neuroendocrine changes, showing that ovarian hormones are not necessary for protection. Second, obese mice exhibit sex differences in DIO-induced inflammation. Microglial activation and peripheral macrophage infiltration is seen in the hypothalami of males, while females are protected from the increase in inflammatory cytokines and do not exhibit microglia morphology changes nor monocyte-derived macrophage infiltration, regardless of the presence of ovarian hormones. Strikingly, the anti-inflammatory cytokine IL-10 is increased in the hypothalami of females but not males. Third, this study posits a potential mechanism of obesity-induced impairment of hypothalamic function whereby obese males exhibit reduced levels of synaptic proteins in the hypothalamus and fewer spines in GnRH neurons, located in the areas exhibiting macrophage infiltration. Our studies suggest that inflammation-induced synaptic remodeling is potentially responsible for hypothalamic impairment that may contribute to diminished levels of gonadotropin hormones, testosterone, and sperm numbers, which we observe and corresponds to the observations in obese humans. Taken together, our data implicate neuro-immune mechanisms underlying sex-specific differences in obesity-induced impairment of the hypothalamic function with potential consequences for reproduction and fertility.

[Neuroendocrine mechanisms controlling the development in puberty. A literature overview]

Puberty is the stage of development in human life, when the hypothalamus-hypophysis-gonad axis is re-activated after quiescence. Humanity has long been concerned with the idea of exogenous and endogenous factors and mechanisms that influence the temporal course of puberty neuroendocrine events. Recent discoveries have helped to understand the functioning of the neuroendocrine system. It has been clarified that kisspeptin plays a key role in puberty and regulation of fertility. However, in the function of the gonadotropin-releasing hormone (GnRH) pulse secretion, besides kisspeptin, neurokinin B, dynorphin neurons other positive and negative signals are involved, guiding the release of hormones of hypophysis gonadotropin. The knowledge of these nerves further enhanced the understanding of GnRH pulsation modulation by endocrine, metabolic and environmental impacts. The authors point out the risk of endocrine disruptors in the physiological course of puberty. The aim of the review is to provide a comprehensive picture of the research results of the physiology of kisspeptin, as the manipulation of kisspeptin signaling has the potential for novel therapies in patients with pathologically low or high luteinizing hormone (LH) pulsatility. Orv Hetil. 2018; 159(29): 1175-1182.

β-arrestins and biased signaling in gonadotropin receptors

Gonadotropin receptors include the follicle stimulating hormone receptor (FSHR) and the luteinizing hormone/choriogonadotropin receptor (LHCGR), both belong to the G protein-coupled receptor (GPCR) superfamily and are essential to reproduction. FSHR is activated by follicle stimulating hormone (FSH) while LHCGR is activated by either luteinizing hormone (LH) or choriogonadotropin (CG). Upon ligand binding, gonadotropin receptors undergo conformational changes that lead to the activation of the heterotrimeric G protein, resulting in the production of different second messengers. Gonadotropin receptors can also recruit and bind β-arrestins. This particular class of scaffold proteins were initially identified to mediate GPCRs desensitization and recycling, but it is now well established that β-arrestins can also initiate Gs-independent signaling by assembling signaling modules. Furthermore, new advances in structural biology and biophysical techniques have revealed novel activation mechanisms allowing β-arrestins and G proteins to control signaling in time and space. The ability of different ligands to preferentially elicit G- or β-arrestin-mediated signaling is known as functional selectivity or biased signaling. This new concept has switched the view of pharmacology efficacy from monodimensional to multidimensional. Biased signaling offers the possibility to separate therapeutic benefits of a drug from its adverse effects. The proof of concept that gonadotropin receptors can be subjected to biased signaling is now established. The challenge will now be the design of molecules that can specifically activate beneficial signaling pathway at gonadotropin receptors while reducing or abolishing those leading to side effects. Such strategy could for instance lead to improved treatments for infertility.

Hypogonadism and male obesity: Focus on unresolved questions

Obesity, increasing in prevalence globally, is the clinical condition most strongly associated with lowered testosterone concentrations in men and presents as one of the strongest predictors of receiving testosterone treatment. While low circulating total testosterone concentrations in modest obesity primarily reflect reduced concentrations of sex hormone binding globulin, more marked obesity can lead to genuine hypothalamic-pituitary-testicular axis (HPT) suppression. HPT axis suppression is likely mediated via pro-inflammatory cytokine and dysregulated leptin signalling and aggravated by associated comorbidities. Whether oestradiol-mediated negative hypothalamic-pituitary feedback plays a pathogenic role requires further study. Although the obesity-hypogonadism relationship is bidirectional, the effects of obesity on testosterone concentrations are more substantial than the effects of testosterone on adiposity. In markedly obese men submitted to bariatric surgery, substantial weight loss is very effective in reactivating the HPT axis. In contrast, lifestyle measures are less effective in reducing weight and generally only associated with modest increases in circulating testosterone. In randomized controlled clinical trials (RCTs), testosterone treatment does not reduce body weight, but modestly reduces fat mass and increases muscle mass. Short-term studies have shown that testosterone treatment in carefully selected obese men may have modest benefits on symptoms of androgen deficiency and body composition even additive to diet alone. However, longer term, larger RCTs designed for patient-important outcomes and potential risks are required. Until such trials are available, testosterone treatment cannot be routinely recommended for men with obesity-associated nonclassical hypogonadism. Lifestyle measures or where indicated bariatric surgery to achieve weight loss, and optimization of comorbidities remain first line.

Update on male reproductive endocrinology

Practitioners of male reproductive and sexual medicine must have an intimate understanding of the physiology of male reproductive endocrinology, as such a knowledge is the cornerstone on which hormonal treatments are based. In this review, we highlight what is known about male reproductive endocrine physiology and the various control mechanisms for the system. We also discuss the limitations of our current understanding of the reproductive physiology. We hope that this review is helpful for male reproductive medicine practitioners in understanding the principles on which hormonal treatments are based.